1. Field of the Invention
The present invention relates, generally, to an improved valve gate for an injection molding system, and more particularly, but not exclusively, the invention relates to means for controlling positioning of a valve stem in a valve gate to improve the efficiency of the molding operation.
2. Background Information
In an ideal situation, the valve stem of the valve gate is open during the fill and hold portion of the molding operation to allow proper filling of the mold and compensate for any shrinkage arising during hold. The stem would then be closed for cooling of the part in the mold, opening of the mold, removal of the part from the mold and the subsequent closure of the mold. The stem would only move to the open position just before the next fill cycle starts. However, this ideal operation can induce defects in the molded part due to heat transfer through contact of the heated valve stem with the cooled gate because of the steel-to-steel contact between the stem and the gate. To avoid these defects, it is common practice to maximize the time that the stem is held in the open position. This practice creates the risk that entrapped pressure in the hot runner channel may cause melt to be pushed in front of the open stem. This melt in front of the stem may become of different consistency, viscosity, and temperature compared to the melt upstream of the stem front face. If this inconsistent melt is then injected into the mold cavity, drool and other defects may occur.
Current injection molding systems have failed to satisfactorily resolve these conflicting requirements of long stem closure time to avoid drool and other related defects and a short stem closure time to avoid defects caused by undesirable heat transfer.
The present invention avoids this conflict by enabling the stem to be partially pulled back in such a way that it is not in contact with the cold gate when in the closed position. The front face of the valve stem is kept inside the nozzle tip land to avoid or at least reduce the likelihood of melt being pushed in front of the stem. Pulling back the stem slightly accomplishes the ideal function of minimizing heat leakage to the gate and encapsulating the melt in the hot runner.
U.S. Pat. No. 5,162,125 to Akselrud et al, patented Nov. 10, 1992, shows a molding device that uses a spring to close a mold gate.
U.S. Pat. No. 5,423,672 to Gordon, patented Jun. 13, 1995, shows a molding device for forming a disc with a hole therein. This patent shows a valve gated hot runner in which the valve stem is moved to an intermediate position by means of a dual piston combination. In the disc molding operation the valve stem is first moved forward to open the valve gate and allow resin to fill the mold cavity. Next the valve stem is partially retracted to block the melt flow and allow a lower part of the valve stem to form the hole in the disc. Finally, the valve stem is fully retracted to pull its hole forming section out of the molded part and to allow the molded part to be ejected conventionally from the core side of the mold.
U.S. Pat. No. 6,214,275 to Catoen et al, patented Apr. 10, 2001, shows a molding device that includes means to move a valve stem into an extended position to assist the ejection of molded parts and remove debris from the gate orifice and nub area of the melt channel.
U.S. Pat. No. 6,228,309 to Jones et al, patented May 8, 2001 shows a molding device that includes apparatus for moving a valve stem between a closed position, a partially open position and a fully open position. The partially open position enables restricted flow of melt to the mold cavity.
None of these references teach the concept of moving a valve stem into an intermediate position between an open and a closed position to minimize cooling of the end of the valve stem by the cooled gate insert.